The Humern empire is vast, unbelievably powerful and (in certain matters) brutally authoritarian.
One such issue is that of reproduction. In order to ensure that all subjects of the Empire remain part of the same species all breeding must be done either by taking a randomly chosen fertilised egg from a Reproduction Center (random in that the egg is picked randomly from a bank of egg/sperm pairings carefully chosen to ensure forward and backward genetic compatibility) or by having the genes of yourself and your partner analysed to ensure you have a reasonable chance of producing compatible offspring (knowing your offspring may well be ‘incompatible’ and this never have children of their own).
Unless a subject goes down one of these two routes they are sterile. They are born sterile, live sterile and can only have sterility taken away at the discretion of the Empire. This is to prevent any breakaway faction or splinter groups being able to reproduce and split into a whole new species (and certainly not so the Empire has an iron grip on the family jewels of all of it’s colonies)
The question is how can such reversible sterility be woven into (for arguments sake) the human genetic code? It is still required that eggs and sperm be available to fill the Reproduction Centers, but the sterility must be status quo without intervention (ie removal of all eggs at birth is not an option as a rebel government might choose to stop doing it)
Answers will be ranked by how biologically complex the changes are, how hard the ‘reversal’ is without proper authorisation, and how low the chance of pregnancy becomes (because it will never be bang on 0, but rebels shouldn’t be able to breed without insane levels of luck).
Details on the biological cause of the sterility are required, even if they’re just an overview. More detail is, in my view, a better answer, but I understand the complexity here (I’m not asking for which proteins need editing, just methods by which this kind of sterility could be effected).
Bonus points: if the changes can be applied to an otherwise healthy human population by means of a retrovirus. Not that the subjects of the Empire won’t willingly submit to such a draconian measure, you understand...